These studies seek to investigate the hematotoxic effects of inhaled benzene at concentrations encountered in environmental settings. We have evidence that exposure of C57B1 mice for six months to 10 ppm. benzene causes a marked reduction in the numbers of certain erythrocyte precursor cells: the erythroid colony forming unit (CFU-E) and the late normoblasts. These results reflect a disruption in erythroid cell production because the CFU-E is one of the two known erythroid stem cells. There appears to be compensation for this disruption because peripheral red blood counts of the affected animals were normal. These findings may be significant from the standpoint of human health because the exposures were conducted at concentrations considered acceptable for occupational exposure to benzene and because current health monitoring of benzene-exposed individuals usually involves only evaluation of peripheral blood counts. We propose to evaluate low level exposures to benzene using the sensitive hematopoietic cell assays to explore: 1. The nature of the dose and temporal responses at low benzene concentrations. 2. Whether precursor cells toxicity observed at low concentrations foreshawdows more extensive hematopoietic damage. 3. The extent to which the toxic effects produced at low concentrations of benzene are reversible. 4. Whether the benzene toxicity would be more severe if hematopoietic system were given additional stress.